Many devices are powered by electrochemical cells, particularly devices for medical use. Examples of such devices include hearing prostheses, neural stimulators, pacers, drug pumps and other devices. Increasingly, these devices use digital processing systems, rather than analog systems which were the standard prior art technique. One feature of digital systems is that the processor used will generally require a certain minimum voltage to operate effectively. If this is not present, the device will fail erratically. To avoid this, a system shutdown voltage level is generally used, at which level the device shuts itself down. This level is often set well above the actual minimum level, to avoid the possibility of error from a dubious power supply. In contrast, prior art analog systems generally fail gradually, with progressively less performance delivered as less voltage is available from the battery. Accordingly, the user has generally more warning of impending device failure.
To take the example of cochlear implants, modern speech processors are controlled by and process speech using a microprocessor. The speech processor also provides power to an induction loop, which via an inductive coupling supplies power and data to an implanted receiver stimulator unit. Although in principle any suitable battery could be used to provide power to such systems, the zinc-air cell is the preferred power source. Such cells are also commonly used for applications such as external hearing aids.
Zinc air cells have several practical advantages. They have a very high energy density, and so can supply a device's requirements for a relatively long period of time relative to their size and weight. They also have a relatively constant power output through most of their life, so that there is little risk of dangerous rapid discharge, for example by shorting. However, if they experience a heavy load, then it is common for the voltage to temporarily sag.
Conventionally, such devices have employed a battery monitor arrangement, whereby the voltage is monitored and if it falls below a certain level, the device is shut down. Such voltage levels are often set at a value which corresponds to a relatively high power demand, so as to prevent anomalous operation due to under voltage. As a result, if even a relatively new battery is subjected to adverse conditions, for example a period of heavy load, the cell voltage may fall below the pre-defined cut-off level and the processor will be shut down. In the field of Cochlear implants such an event is inconvenient and has potentially serious implications. After shut-down, the user must reset the speech processor by re-starting it, and hence the user is disconnected for a time from their hearing environment. Similar problems can arise with other battery powered digital systems, where short term conditions cause a temporary reduction in the voltage of the power supply.
It is an object of the present invention to provide an improved battery monitoring arrangement in order to improve the performance of battery powered devices.